The Collaborative International Dictionary of English v.0.48:

Investigator \In*ves"ti*ga`tor\, n. [L.: cf. F. investigateur.]
   One who searches diligently into a subject.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Researcher \Re*search"er\ (-[~e]r), n.
   One who conducts research. In the field of scientific
   research, also called an investigator or scientist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

investigator
    n 1: a scientist who devotes himself to doing research [syn:
         research worker, researcher, investigator]
    2: someone who investigates
    3: a police officer who investigates crimes [syn: detective,
       investigator, tec, police detective]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "investigator":
   Bow Street runner, FBI, FBI agent, Federal, G-man, Secret Service,
   Sherlock, Sherlock Holmes, T-man, detective, dick, examinant,
   examiner, eye, fed, gumshoe, hawkshaw, hotel detective,
   house detective, house dick, inquiry agent, inspector, narc,
   observer, operative, plainclothesman, police detective,
   private detective, private investigator, revenuer, scrutineer,
   sleuth, store detective, tec, tester, treasury agent, visitator,
   visitor